A wireless mesh network, WMN, comprises a plurality of mesh nodes organized in a mesh topology. Here, each mesh node is also some sort of provider by forwarding data to the next mesh node. The network infrastructure is decentralized and simplified because each mesh node only needs to be able to transmit to a neighbouring mesh node. Such wireless mesh networks could allow people living in remote areas and small businesses operating in rural neighbourhoods to connect their networks together for affordable internet connections.
Wireless mesh network can be implemented with various wireless technologies including 802.11, 802.15, cellular technologies and Bluetooth. Bluetooth mesh networking is standardized by the Bluetooth Special Interest Group, SIG, and the first release of Bluetooth Mesh was released in July 2017. The solution is based on flooding using broadcasting over a set of shared channels referred to as the advertisement channels.
A node acting as a relay in a Bluetooth mesh network scans the advertisement channels for mesh messages. When a message is detected and received, the node checks to see if it is the destination of the message. If the node is not the destination of the node, the node checks if it has already received and forwarded the message. If yes, the message is discarded. If not, the message is forwarded in the mesh network by re-transmitting it over the advertisement channels so that the neighbours of the node can receive it. Typically some random delay is introduced before forwarding the message to avoid collisions. By means of this distributed mechanism, the message is forwarded from node to node in the network so that the message arrives at the destination.
The advertising mechanism works such that a given message is also repeated in three separate physical channels, denoted as advertising channels, opportunely spaced in frequency to guarantee robustness to frequency selective fading and interference. On the receiver side, nodes must scan all channels periodically. Therefore, it is not possible to predict in which channel a message is received. To lower the probability of message collisions on all advertising channels, the Bluetooth Mesh specification recommends randomizing the gap within consecutive messages within an advertising event. Up to 10 ms delay is allowed between each of the messages in the advertising event.
The features within the Bluetooth SIG specification enable many nodes in the mesh network to be battery-powered or to use techniques such as energy harvesting. If a node cannot scan continuously, then it is possible that it will not receive mesh messages that it should be processing. In order to facilitate the use of such nodes within a network, the concept of “Friendship” is used within the Bluetooth Mesh network by Low power nodes.
Friendship is first established and initiated by the Low Power Node. Friendship is a special relationship between a Low Power node and one neighbouring “Friend Node”. These nodes must be within a single hop of one another and in the same subnet. Once friendship is established, the Friend node performs several actions that help reduce the power consumption of the Low Power node. The Friend node maintains a Friend queue for the low power node, which stores incoming messages addressed to the Low Power node.
The Friend node delivers those messages to the Low Power node when requested by the Low Power node. Also, the Friend node delivers security updates to the Low Power node. Furthermore, a Friend node may be friends with multiple Low Power nodes, but a Low Power node can only be friends with a single friend node.
Controllers, or nodes in mesh network, are typically implemented with a single-mode operation for the transmission of advertisements and today's solutions do not consider the type of advertising data to be sent. The recommendation of randomizing the gap between consecutive messages within an advertising event introduces advantages for the successful end to end delivery of mesh messages, but also increases the average delay for successful transmission, thus increasing the energy consumption at Low Power nodes.
As an example, assuming messages are sent back to back, the duration of an advertising triplet transmission is less than 1 ms. When introducing random delay between messages, the advertising triplet transmission time can exceed 20 ms.